Session B17: Focus Session: Carbon Nanotubes: Electronic and Transport Properties

Carbon nanotube high-performance logic technology -- challenges and current progress

In the last four decades, we have witnessed a tremendous information technology revolution originated from the relentless scaling of Si complementary metal-oxide semiconductor (CMOS) devices. CMOS scaling provides ever-improved transistor performance, density, power and cost, and will continue to bring new applications and functions to our daily life. However, the conventional homogeneous scaling of silicon devices has become very difficult, firstly due to the unsatisfactory electrostatic control from the gate dielectric. In addition, as we look forward to the technology nodes with sub-10 nm channel length, non-Si based channel materials will be required to provide continuous carrier velocity enhancement when the conventional strained-Si techniques run out of steam. Single-walled carbon nanotubes are promising to replace silicon as the channel material for high-performance electronics near the end of silicon scaling roadmap, with their superb electrical properties, intrinsic ultrathin body, and nearly transparent contact with certain metals. This talk discusses recent advances in modeling and experimental works that reveal the properties and potential of ultra-scaled nanotube transistors, separation and assembly techniques for forming nanotube arrays with high semiconducting nanotube purity and tight pitch separation, and engineering aspects of their implementation in integrated circuits and functional systems. A concluding discussion highlights most significant challenges from technology points of view, and provides perspectives on the future of carbon nanotube based nanoelectronics.   

Measurement of the resistance induced by a single atomic impurity on a (7,6) semiconducting carbon nanotube: scattering strength of individual potassium atoms as a function of gate voltage

Despite many years of research, no measurements have been performed to determine resistance induced by impurities in carbon nanotubes. Over the last few years, we have developed a capability to measure the resistance induced by a single impurity atom on nanotubes with known chirality. Using this capability, we measured the resistance induced by an individual potassium atom on a (7,6) semiconducting carbon nanotube. The ``atomic'' resistance of potassium is found to be in the kohm range and has a strong dependence on the applied gate voltage. The scattering strength of the p-type (valence band) channel is approximately 20 times greater than that of the n-type (conduction band) channel. We integrate our atomically-controlled experimental result to a numerical recursive Green's function technique, which can precisely model the experiment, to understand the measured ``atomic'' resistance and the asymmetry.   

Unusual conductance suppression in metallic carbon nanotubes

Clean metallic carbon nanotube usually show a finite ``dip'' in the conductance as a function of gate voltage. We have observed an extra conductance suppression as larger gate voltages are applied, forming a hump-like shape in the conductance versus gate voltage curve. We have performed numerical calculations using the recursive Green's function technique on a tight-binding model to identify the origin of the hump. Our results show that the hump formation is associated with the unique combination of the linear dispersion of metallic carbon nanotubes and the presence of extrinsic impurities such as adsorbates. The width and position of the hump strongly depend on the densities of positive and negative impurities but show no dependence on the shape of the impurity potential, indicating why the hump is a rather ubiquitous phenomenon for metallic nanotubes.   

Electrical contact to carbon nanotubes encapsulated in hexagonal boron nitride

Hexagonal boron nitride has been an excellent platform for low dimensional materials. We have fabricated ultra clean single-walled carbon nanotube(SWNT) devices encapsulated in hexagonal boron nitride by a dry transfer technique. Contacts to the SWNTs were made by reactive ion etching to expose the ends of SWNTs, followed by metal deposition. Ohmic contacts to SWNTs were achieved. We will discuss the quality of the contacts using different combinations of metals and present latest transport data.   

One-dimensional Poole-Frenkel conduction in the single defect limit

Theory predicts a range of phenomena in disordered one-dimensional (1D) conductors, but few physical systems exist for direct comparison with experimental observation. Recently, we demonstrated an electrochemical technique for adding a single, isolated point defect to a single-walled carbon nanotube (SWNT) in a field effect transistor device. A point defect, surrounded on either side by quasi-ballistic, semi-metallic SWNT is an ideal system for investigating disorder in 1D. Here, transport and Kelvin probe force microscopy independently demonstrate high-resistance depletion regions that can extend from 0.3 - 2.0 $\mu $m wide surrounding the defect site. The defect assists tunneling through this depletion region via a modified, 1D version of Poole-Frenkel conduction. The width of the depletion region is found to depend sensitively on SWNT diameter and carrier density, as expected for a molecular scale wire. Surprisingly, conduction is well described by the 1D Poole-Frenkel model over a wide range of temperature from 77 - 300 K and over a wide range of source-drain bias from 0.1 - 2.0 V.   

Electronic Properties of Graphene and Single Wall Carbon Nanotubes in the Presence of Hexagonal BN Islands

Controlled chemical doping with Boron (B) and Nitrogen (N) is a promising approach for electronic band engineering of carbon-based materials. Based on the first-principles Density Functional Theory (DFT) calculations, we investigated the effect of hexagonal BN islands on the electronic properties of graphene as well as carbon nanotubes. Our results are in agreement with recent experimental observations that hexagonal BN islands in graphene open a sizable electronic band gap in graphene. Our detailed analysis show that, the electronic properties not only depend on the impurity concentration, but also depend on the geometrical pattern of impurity atoms in the honeycomb network of C atoms in graphene. We identified interesting symmetry properties, which controls the band gap of the system. A similar behavior was observed in the effect of hexagonal BN islands on electronic properties of zigzag single wall carbon nanotubes (SWCNT). Using the Density functional Perturbation Theory, we also investigated the vibrational properties of SWCNTs with hexagonal BN islands, which confirm the stability of these systems.   

Ambipolar Transistors with Heterostructures of Single-Walled Carbon Nanotubes and Zinc Tin Oxide

The unique operation of ambipolar thin-film transistors (TFTs), in which both electrons and holes can be injected and transported in a single device, have attracted significant attention since it was first demonstrated in mid-1990s. In addition to their unique operation, these devices have great potential in complementary-like circuits and novel light emitting transistors. Single-walled carbon nanotubes (SWCNTs) exhibit ambipolar behavior intrinsically; however, SWCNTs under ambient conditions show strong p-type behavior due to adsorption of oxygen and moisture from air. In this work, we will discuss the performance characteristics of ambipolar TFTs with heterostructures of a network of SWCNTs and amorphous zinc tin oxide. These TFTs exhibit well-balanced electron and hole mobilities under ambient conditions, and both carriers are injected through Ti/Au contacts without large injection barriers. Charge transport in this material system will be described. In addition, complementary-like inverters which are composed of two ambipolar TFTs will be demonstrated.   

Zero-Dimensional Electrical Contact to a One-Dimensional Material

Recent work has shown that one-dimensional contacts can be made to two-dimensional graphene using boron nitride encapsulated graphene structures along with an etch process[1]. Here we report the encapsulation of carbon nanotubes, a one-dimensional material, between layers of boron nitride. By etching the edges we are able to use only the zero-dimensional ends of the carbon nanotubes to yield high-quality electrical contacts. This end-contact geometry along with an encapsulated nanotube provides possibilities for the realization of more complex nanotube heterostructure devices. [1]L. Wang et al., Science 342, 614-617 (2013).   

Sensitizing Carbon Nanotube Transistors for Single Molecule Sensor Applications

Recent work has demonstrated single-charge sensitivity in two types of carbon nanotube transistors. In one case, a two-level system near the nanotube or noncovalently attached to the nanotube perturbs the current electrostatically. In a second case, a sidewall defect or other covalent modification sensitizes one site along the conductor. Comparative research has helped reveal differences in the transduction mechanisms of the two cases and provides design rules for maximizing reliable signals for sensing applications. The covalent modifications are not mere perturbations and they are far more sensitive than noncovalent attachments, for example. However, the new degrees of freedom that accompany covalent disorder often have similar energy scales, leading to multiple independent fluctuations that degrade the overall signal-to-noise. Noncovalent sensitization generally produces a smaller signal amplitude in a background of other low-energy fluctuators, but a well-designed noncovalent linker can result in a highly predictable signal amplitudes. Furthermore, noncovalent fabrication methods are scalable, so that wafer-scale arrays of molecular sensors are most likely to follow this path.   

Defect Screening Effects of Fluoropolymer Capping in Single Walled Carbon Nanotube Transistors

One of the most promising uses of single walled carbon nanotubes (SWCNTs) is as active channel semiconductor materials in field-effect transistors (FETs). Recent advances in the availability of highly sorted semiconducting SWCNT source material and in printing such nanotubes to realize high-performance thin-film transistors make them very promising candidates for printed electronics. In this presentation, we report on the substantial improvements in the characteristics of SWCNT FET devices and circuits comprised of these devices by the use of coatings of the fluoropolymer containing copolymer, PVDF-TrFE. The origins of these improvements may be attributed to the polar nature of C-F bonds and the local organization of the fluoropolymer at the interfaces with the SWCNTs so as to partially neutralize charged defects. This hypothesis was tested by the experiments using a number of vapor phase polar molecules which produce similar effects on the FET characteristics. The polar vapor experiments show that dipoles can partially neutralize residual charges arising from defects/impurities. The dipole present in polar molecules adopts an orientation that tends to cancel the effects of the charged defect/impurity from the perspective of mobile charges in the SWCNTs.   

Low Energy Dissipation Nano Device Research

The development of research on energy dissipation has been rapid in energy efficient area. Nano-material power FET is operated as an RF power amplifier, the transport is ballistic, noise is limited and power dissipation is minimized. The goal is Green-save energy by developing the Graphene and carbon nantube microwave and high performance devices. Higher performing RF amplifiers can have multiple impacts on broadly field, for example communication equipment, (such as mobile phone and RADAR); higher power density and lower power dissipation will improve spectral efficiency which translates into higher system level bandwidth and capacity for communications equipment. Thus, fundamental studies of power handling capabilities of new RF (nano)technologies can have broad, sweeping impact. Because it is critical to maximizing the power handling ability of grephene and carbon nanotube FET, the initial task focuses on measuring and understanding the mechanism of electrical breakdown. We aim specifically to determine how the breakdown voltage in graphene and nanotubes is related to the source-drain spacing, electrode material and thickness, and substrate, and thus develop reliable statistics on the breakdown mechanism and probability.